1. Field of the Invention
The present invention relates to a liquid discharge head and a liquid discharge device.
2. Description of the Related Art
A liquid discharge head for use in inkjet recording has a plurality of discharge ports and a plurality of energy generating elements for generating thermal energy to be utilized for discharging ink from the discharge ports. Such an energy generating element contains a heat generation resistant layer and an electrode for supplying electric power to the resistant layer. By covering the energy generating element with an insulating layer containing an insulating material, insulation between the ink and the energy generating element is secured. The ink is sharply heated by thermal energy generated by driving such energy generating elements, whereby the ink undergoes film boiling, so that bubbles are formed. Then, the ink is discharged to a recording medium by pressure accompanied with these bubbles, and thus recording is performed.
A heat acting portion in which the heat generated by the energy generating elements is transmitted to the ink is subjected to high temperature due to heating of the heat generation resistant layer and also subjected in a complex manner to a physical action, such as impact of cavitation accompanied with expansion and contraction of bubbles and a chemical action by the ink. Therefore, in order to protect the energy generating element and the insulating layer, a protective layer is provided at a position corresponding to the energy generating element. Usable as a material of such a protective film are tantalum and a platinum group (iridium, ruthenium, and the like) resistant to the impact caused by cavitation and the chemical action caused by ink. In particular, a film of the platinum group, such as iridium and ruthenium, is a very stable film and the film has resistance to ink and has strong resistance to the impact caused by cavitation. Therefore, the materials are useful from the viewpoint of reliability and extension of life-span of the head.
However, it is known that such materials are hardly chipped even when cavitation arises, and therefore a hardly-soluble substance (kogation) generated due to thermal decomposition of a substance in ink is likely to adhere to the surface of the protective layer, and is gradually deposited. When such kogation adheres to the heat acting portion, the energy generated by the energy generating element is not sufficiently transmitted to ink, resulting in unstable discharge.
Japanese Patent Laid-Open No. 2008-105364 discloses applying a voltage to the protective layer to cause an electrochemical reaction between the ink and the protective layer to elute about several nanometers of the surface of the protective layer to thereby remove the kogation deposited on the protective layer. FIG. 7 is a cross-sectional schematic view of the periphery of an energy generating element of a liquid discharge head of Japanese Patent Laid-Open No. 2008-105364. An energy generating element 1108 contains a heat generation resistant layer 1104 and an electrode layer 1105 is covered with an insulating layer 1106 containing a silicon nitride film or the like. At the position corresponding to the energy generating element 1108, a protective layer 1107 containing iridium and ruthenium and a wiring layer 1109 supplying electric power to the protective layer 1107 are provided. It is disclosed that kogation deposited on the energy generating element 1108 can be removed by applying a voltage in such a manner that the protective layer 1107 serves as an anode using the wiring layer 1109 and a counter electrode 1111 to elute about several nanometers of the surface of the protective layer 1107.
However, as is understood from FIG. 7, the surface of the insulating layer 1106 corresponding to the energy generating element 1108 of the liquid discharge head disclosed in Japanese Patent Laid-Open No. 2008-105364 has a convexo-concave shape due to the electrode layer 1105. Since the protective layer 1107 is generally formed using a film formation technique, such as a sputtering method, the film thickness becomes small at a level difference portion or an inclined portion. More specifically, the film thickness of the protective layer 1107 at the position corresponding to the boundary portions of the electrode layer 1105 and the heat generation resistant layer 1104 (hereinafter also referred to as a tapered portion) is smaller than that of a flat portion above the energy generating element, and the film thickness is only about 50 to 60% of the film thickness of a region above the energy generating element. The film quality of the boundary portions, i.e., the tapered portions of the protective layer 1107 corresponding to the end surfaces apart from each other of a pair of electrode layers 1105, is bad as compared with the film quality of the flat portion.
When a kogation removal operation is performed using such a liquid discharge head, an electrochemical reaction rapidly proceeds in the protective layer of the portions corresponding to the boundary portions, so that the protective layer of the portions is very rapidly eluted. Therefore, when the kogation removal operation is repeated, the film thickness of the protective layer at the portions corresponding to the boundary portions is further reduced as compared with the film thickness of the flat portion. Therefore, a crack and the like arise due to cavitation in the protective layer with the tapered portions as the starting point, which causes a problem such that the energy generating element cannot be sufficiently protected.